The present invention relates to a color separation optical reading apparatus to be applied to, for example, a facsimile, a scanner, a copying machine, and so on.
In the color separation optical reading apparatus of the kind described above, generally, picture information of an object is read-in through separation into color components of B (blue, 450 nm), G (green, 525 nm), and R (red, 620 nm), and the thus read-in components of information are composed to thereby obtain a color picture. Accordingly, as an imaging optical system in the above-mentioned apparatus, it is necessary to use a lens system capable of performing separation of color information. In the imaging lens system, however, chromatic aberration may be generally generated depending on the wavelength. In this regard, the above-mentioned imaging lens system is different from the conventional monochromatic optical system in which no color separation as well as no high accuracy for reading have been required.
Since the imaging lens system is required to have a large aperture, the depth of focus becomes narrow so that the three colors, B, G and R are deteriorated in performance owing to the residual chromatic aberration in color separation.
It is possible, ideally, to eliminate such chromatic aberration by displacing at least one of the object, the imaging lens system, and the light reception member in the direction of the optical axis with high accuracy in accordance with the selected wavelength of B, G or R and in accordance with the amount of chromatic aberration. However, the control system for the above-mentioned displacement is mechanically complicated and therefore expensive. Accordingly, conventionally, such a simple color separation optical reading apparatus as illustrated in FIG. 1 has been used.
FIG. 1 shows a structure of a conventionally used color scanner. A lighting source 12 and scanning mirrors 13 and 14 are provided below transparent plate glass 11 on which an original (an object) 0 is mounted. The lighting source 12 and the scanning mirrors 13 and 14 are made to scan the original from a position shown by a solid line to another position shown by a broken line in FIG. 1, and the illumination light reflected from the original 0 is reflected by the scanning mirrors 13 and 14 so as to impinge onto a line sensor 17 such as a CCD (i.e., a light receiving member) through a wavelength selection filter (wavelength selection means) 15, and an imaging optical system 16. That is, the whole of the original 0 is read by the line sensor 17. In the conventional apparatus described above, in order to perform color separation, the wavelength selection filter 15 disposed in the optical path between the imaging optical system 16 and the line sensor 17 is switched corresponding to successive ones of the three colors, G, B and R. Alternatively, a wavelength selection means is disposed between the lighting source 12 and the original 0 (including the case in which the wavelength of the lighting source is changed over) and arranged to be switched corresponding to successive ones of the three colors, R, G and B, similarly to the former case. In the respective conditions, the lighting source 12 and the scanning mirrors 13 and 14 are made to scan the original to thereby obtain picture information separated into colors of G, B, and R. In the conventional apparatus, however, there is the following problem.
In FIG. 2, the ordinate axis represents the MTF of the imaging lens for each of the wavelengths of the colors B, G and R and the abscissa axis represents a defocus quantity. Considering the wavelength of the color B, defocus amounts of .DELTA.fB.sub.G and .DELTA.fB.sub.R are caused with respect to the wavelengths of the colors G and R, owing to the chromatic aberration of the imaging lens. In the conventional apparatus, in order to minimize the defocus quantity, the line sensor 17 of a CCD or the like is located at a position S, as shown in FIG. 2, at which the defocus is minimum with respect to the wavelengths of the colors B, G and R, so that the MTF values of the colors B, G and R can be obtained in an average manner. However, in the conventional apparatus in which only the position of the line sensor 17 is set to correct defocus caused by chromatic aberration, an optimum MTF value with respect to each wavelength cannot be obtained and therefore a sufficient output cannot be obtained. Further, the smaller the F value of the imaging lens system is, the narrower the width of the MTF value with respect to defocus, so that the MTF value is further lowered at an average focal position.